Lithium ion batteries are secondary batteries that can be used repeatedly by charging, generally constituted from a cathode that includes a metal oxide such as lithium cobalt oxide (LiCoO2) or the like as the cathode active material, an anode that includes a carbon material such as graphite or the like as the anode active material, and an electrolyte solution that uses solution of electrolyte in a carbonate or the like. Furthermore, in lithium ion batteries, discharging and charging can occur through the migration of lithium ions between a cathode and an anode.
Compared to batteries such as nickel batteries, nickel-hydrogen batteries and the like, lithium ion batteries have a greater energy density and a higher discharge voltage. For this reason, lithium ion batteries can be designed to be miniaturized and light-weight. Moreover, lithium ion batteries also combine advantages such as no memory effect, superior charge/discharge cycle characteristics, and the like. For this reason, lithium ion batteries have become essential for mobile devices, such as notebook computers, cellular telephones, portable game devices, digital cameras, electronic organizers, and the like, for which miniaturization and being lightweight are important product values. Recently, in addition to being miniaturized and light-weight, mobile devices become more highly functionalized with each passing year, for example, such as being equipped with One-Seg functions. For this reason, batteries with more advantages such as higher capacity and higher performance are required in mobile devices.
Thus, in recent years, the high storage/discharge capacity of lithium is expected to be useful in tin and/or tin alloy or silicon and/or silicon alloy as an anode active material for implementing higher capacity batteries. Furthermore, the anode is generally obtained by employing an anode mix slurry that contains an anode active substance and a binder to form an anode layer on the surface of an anode current collector body such as copper or the like. Here, the binder binds the active substance to another active substances or the active substance to the current collector body, and is essential for preventing detachment of the active substance from the current collector body.
In addition, examples of binder compositions used in forming a carbon material anode frequently used in industry include an N-methyl-2-pyrrolidone (NMP) solution of poly vinylidene fluoride (PVDF) and the like.
However, while PVDF is excellent as a binder that combines carbon materials to each other, it exhibits poor adhesion to a current collector body metal such as copper or the like. For this reason, carrying out repeated charging and discharging in batteries in which PVDF is employed as a binder reduces the battery capacity because the carbon materials as the active materials are detached from the current collector body. In other words, there is a problem with shortened cycle life.
Additionally, when silicon and/or silicon alloy is employed as the main component of the anode active substance, the volume of this anode active substance expands by 3- to 4-fold during charging. Consequently, when the binder used in such cases does not have sufficiently high adhesive strength as in conventional binders, cracks are produced in the active substance, which ultimately becomes finely pulverized due to the repeated expansion and contraction caused by charging/discharging. As a result, there is a problem with such batteries in that ultimately the charge-discharge cycle characteristics rapidly deteriorate.
One of the methods proposed for solving such a problem is the use of a polyimide resin as a binder (for example, see: Japan Laid-open Patent Application Publication No. JP-A-H11-158277 (1999), PCT International Application Publication No. WO 2004/004031, and the like). However, when polyimide resin is used as a binder, there is a problem that the active substance can easily become completely coated, and the formation of a stable electrode interface (SEI) on the anode surface is readily inhibited.